A problem in the above described arrangement is that the system operator manually determines the neighbours of each base station's sub-cells. The neighbours must be determined because a sub-cell and its neighbour cell cannot use one and the same dynamically selected frequency/time slot combination because transmitters occupying the same frequency and time slot would cause too much co-channel interference for each other. In addition, handover between sub-cells becomes difficult if the neighbours are not determined. A manual determination of neighbours requires time-consuming manual measurements of reception powers, performed within the sub-cells. A large number of measurements may be required. When a new sub-cell is added to the system, or the position of a sub-cell antenna unit is changed, manual determination of neighbours will have to be repeated. This makes the system significantly more difficult to optimise and to expand, and increases the operating costs of the system.